m3u8_edit:add_subtitles

Generates an `EXT-X-MEDIA` entry with the specified `GROUP-ID`, then adds this `GROUP-ID` as the value for
`SUBTITLES` for all the `EXT-X-STREAM-INF` entries in the HLS playlist.

Only supports master playlists which don't yet include any subtitles.

Input parameters description:

* name
  used as the value for `NAME`

* uri
  used as the value for `URI`

* language
  used as the value for `LANGUAGE`

* autoselect
  if true, `AUTOSELECT` is set to `YES`. Implicitly set if `DEFAULT` is true

* default
  if true, `DEFAULT` is set to `YES`

* forced
  if true, `FORCED` is set to `YES`

* closed_captions
  mark the entry as closed captions. By setting this to `true`, this entry will have the following
  Uniform Type Identifiers [UTI] set in the `CHARACTERISTICS`:

    *public.accessibility.transcribes-spoken-dialog
    *public.accessibility.describes-music-and-sound

* characteristics
  a list of strings - Uniform Type Identifiers [UTI] - to specify in the `CHARACTERISTICS` they are de-duplicated before
  written to the file. If `closed_captions` set to true, this will include two pre-set UTIs

Example:

    {
      "tool": "m3u8_edit:add_subtitles",
      "parameters": {
        "input_master_playlist": "index.m3u8",
        "output_master_playlist": "index_new.m3u8",
        "group_id": "subtitles",
        "inputs": [
            {
                "name": "fr-FR",
                "uri": "subs-01.m3u8",
                "language": "fr-FR",
                "autoselect": true,
                "default": true,
                "forced": false,
                "closed_captions": true,
                "characteristics": [
                    "my.custom.uti.01",
                    "my.custom.uti.02"
                ]
            },
            {
                "name": "es-ES",
                "uri": "subs-01.m3u8",
                "language": "es-ES",
                "autoselect": true,
                "default": false,
                "forced": false,
                "closed_captions": false,
                "characteristics": [
                    "my.custom.uti.01",
                    "my.custom.uti.02"
                ]
            }
        }
      }
    }